drm/exynos: add mode_fixup callback to exynos_drm_crtc_ops

crtc::mode_fixup callback is required by crtcs which internally use
a different mode than requested by user - case of Exynos Mixer.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Signed-off-by: Inki Dae <inki.dae@samsung.com>